The present invention relates to an electric power steering apparatus using an electric motor as a source for generating a steering assist force.
Controlling the steering of a vehicle is carried out by transmitting the rotation operation of the steering wheel disposed inside a vehicle interior to a steering mechanism disposed outside the vehicle interior in order to steer the steering control wheels to be steered (generally the front wheels of the vehicle).
FIG. 1 is a sectional view showing a conventional electric power steering apparatus, and FIG. 2 is a sectional view showing the reduction mechanism portion thereof.
As shown in FIG. 1, the conventional electric power steering apparatus of a vehicle for example comprises a first steering shaft 101 connected to a steering wheel 100, a second steering wheel 103 whose upper end portion is coaxially connected to the lower end portion of the first steering shaft 101 via a torsion bar 102 and whose lower end portion is connected to a steering mechanism linked to wheels, a torque sensor 104 for detecting torque applied to the first steering shaft 101 depending on torsion generated in the torsion bar 102 by the rotation of the steering wheel 100, a steering assist motor 105 driven on the basis of the detection result by the torque sensor 104, and a reduction mechanism having a worm 106 and a worm wheel 107 and connected to the output shaft of the motor 105 to reduce the rotation speed of the output shaft and to transmit the rotation to the second steering shaft 103. In this electric power steering apparatus having the above-mentioned configuration, the operation of the steering mechanism depending on the rotation of the steering wheel 100 is assisted by the rotation of the motor 105, whereby the steering effort of the driver can be reduced.
As shown in FIG. 2, the worm 106 constituting the reduction mechanism is supported in the fitting holes of a housing 110 via a pair of rolling bearings 108a and 108b. The second steering shaft 103 provided with the worm wheel 107 is supported in the fitting holes of the housing 110 via a pair of rolling bearings 109a and 109b as shown in FIG. 1, and is prevented from moving in the radial and axial directions thereof.
In the case when such reduction mechanism is used, the worm 106, the worm wheel 107, the rolling bearings 108a, 108b, 109a and 109b, the second steering shaft 103 and the housing 110, machined so that the distance between the rotation centers of the worm 106 and the worm wheel 107 coincides with the distance between the centers of the fitting holes for the rolling bearings 108a and 108b and the centers of the fitting holes for the rolling bearings 109a and 109b within an allowable range, are assembled selectively in order to reduce the amount of the backlash at the meshing portion of the worm 106 and the worm wheel 107. However, this assembly work takes a long time. In addition, the amount of the backlash increases when the amount of the wear at the teeth of the worm 106 and the worm wheel 107 increases. Countermeasures for these problems have been demanded.
In addition, another electric power steering apparatus is known. This apparatus is configured to reduce the amount of the backlash at the meshing portion of the worm 106 and the worm wheel 107 by providing rubber rings between the outer peripheral surfaces of the bearings 108a and 108b for supporting the worm 106 connected to the output shaft of the motor 105 and the fitting holes of the housing 110, and by energizing the worm 106 toward the worm wheel 107 (hereafter referred to as a first radial direction Y) by virtue of the elastic restoring forces of the rubber rings. One of the pair of fitting holes into which the bearings 108a and 108b are fitted is made eccentric with respect to the other.
However, the tooth lead of the worm wheel 107 of the reduction mechanism is inclined in its rotation direction with respect to its rotation center line. For this reason, when rotation torque is applied from the worm 106 to the worm wheel 107, that is, when steering is assisted by the rotation of the motor 105, the worm 106 is pressed relatively strongly so as to be moved in its radial direction (hereafter referred to as a second radial direction X) along the tooth lead of the worm wheel 107. In the case of the conventional electric power steering apparatus configured to reduce the amount of the backlash by using the rubber rings as described above, the elastic restoring force (spring constant) of the rubber ring is constant along the entire circumference thereof. This causes problems described below.
When the elastic restoring force of the rubber ring is relatively small the rotation torque applied to the meshing portion of the worm 106 and the worm wheel 107 (the torque applied to the steering shaft) can be made relatively small. However, the amount of the movement of the worm 106 in the second radial direction X becomes relatively large, and the worm 106 is in danger of being damaged.
In addition, when the elastic restoring force of the rubber ring is relatively large, the amount of the movement of the worm 106 in the second radial direction X can be made very small. However, the rotation torque applied to the meshing portion of the worm 106 and the worm wheel 107 becomes relatively large, and the rotation performance of the worm 106 becomes worse. This worsens the return of the steering wheel and also worsens the feeling of the steering wheel when steering is neutral.
Furthermore, as shown in FIG. 2, both end portions of the worm 106 constituting the reduction mechanism are supported in the fitting holes of the housing 110 via the pair of rolling bearings 108a and 108b. A screw ring 111 screwed into a screw hole formed so as to be connected to the fitting hole of the housing 110 is made contact with the outer ring of one of the rolling bearings, that is, 108a. By rotating the screw ring 111, the outer ring and the inner ring of the rolling bearing 108a are moved relatively in its axial direction, thereby eliminating thrust gaps at the pair of rolling bearings 108a and 108b, and preventing the worm 106 from rattling in its axial direction. Moreover, the second steering shaft 103, on which the worm wheel 107 is provided, is supported in the fitting holes of the housing 110 via the pair of rolling bearings 109a and 109b, thereby being prevented from moving in its radial and axial directions.
However, when preventing the worm 106 from rattling in its axial direction by using the screw ring 111, it is necessary to use error absorption means for absorbing an adjustment error owing to the rotation operation of the screw ring 111 on the side of the screw ring 111 in order to securely eliminate the thrust gaps at the pair of rolling bearings 108a and 108b. Besides, since the outer ring of the rolling bearing 108a is directly pressed by the rotation operation of the screw ring 111, the rolling bearing 108a is in danger of being preliminary pressurized more than necessary owing to a slight rotation error of the screw ring 111. In this case, the rotation torque of the worm 106 increases, thereby increasing the frictional resistance of the entire steering assist mechanism. This worsens the return of the steering wheel and also worsens the feeling of the steering wheel when steering is neutral. Countermeasures for this problem have been demanded.
In addition, as means for reducing the amount of the backlash at the meshing portion of the worm 106 and the worm wheel 107, a configuration is designed, set and manufactured so that a minus gap, i.e., a minus tolerance, is generated in the radial direction at the meshing portion of the worm 106 and the worm wheel 107 by making the distance between the centers of the fitting holes for the rolling bearings 108a and 108b and the centers of the fitting holes for the rolling bearings 109a and 109b of the housing 110 smaller than the distance between the rotation centers of the worm 106 and the worm wheel 107. Furthermore, this configuration is provided around the outer periphery of the rolling bearing 108, with an inner side ring fitted onto the rolling bearing 108, an outer side ring fitted into the fitting hole of the housing 110 and a ring-shaped elastic member for making the worm 106 eccentric with respect to the worm wheel 107. With this configuration, the meshing portion of the worm 106 and the worm wheel 107 is preliminary pressurized by the elastic restoring force of the elastic member. Hence, it is possible to absorb an allowable amount of eccentricity owing to the difference in the distance.
However, in the electric power steering apparatus wherein the worm 106 is made eccentric with respect to the worm wheel 107 by the elastic restoring force of the elastic member, the elastic restoring force of the elastic member is required to be made relatively small in order to make the rotation torque applied to the meshing portion of the worm 106 and the worm wheel 107 relatively small and to apply a necessary minimum preliminary pressure to the meshing portion of the worm 106 and the worm wheel 107. Hence, the worm 106 is pressed in its radial direction so as to be separated from the worm wheel 107 by a meshing reaction force applied to the meshing portion of the worm 106 and the worm wheel 107. As a result, the worm 106 is in danger of moving beyond the allowable amount of eccentricity owing to the minus tolerance of the distance between the rotation centers of the worm 106 and the worm wheel 107 and worsening the meshing with the worm wheel 107.
The present invention is intended to provide an electric power steering apparatus capable of solving the above-mentioned problems.
An electric power steering apparatus in accordance with a first invention comprises a driving gear rotatably supported by bearing inside a housing and rotating interlocked with the rotation of a steering assist motor, and a driven gear meshing with the driving gear and connected to a steering mechanism so that steering is assisted by the rotation of the motor, characterized in that elastic members are provided on the outer periphery of the bearing to restrain the bearing from moving in the radial direction thereof, and that the restraining forces of the elastic members are made different depending on the position in the peripheral direction thereof.
In the first invention, the elastic members provided on the outer periphery of the bearing supporting the driving gear inside the housing are disposed so that a portion, wherein the force for restraining the movement of the bearing in the radial direction is small, is positioned in a first radial direction Y, and so that another portion, wherein the force for restraining the movement in the radial direction is large, is positioned in a second radial direction X. With this configuration, the amount of backlash at the meshing portion can be reduced by the elastic restoring forces of the elastic members. In addition, rotation torque applied to the meshing portion of the driving gear and the driven gear can be made relatively small. Furthermore, the amount of the movement of the driving gear in the second radial direction X can be made very small, whereby the driving gear can be prevented from being broken.
In accordance with a second invention, an electric power steering apparatus as set forth in the first invention is characterized in that the elastic members are disposed on one radial direction side portion and the other radial direction side portion of the bearing.
In the second invention, the elastic members are disposed on one radial direction side portion and the other radial direction side portion of the bearing in the second radial direction. With this configuration, the amount of the backlash at the meshing portion can be reduced by the elastic restoring forces of the elastic members. In addition, rotation torque applied to the meshing portion of the driving gear and the driven gear can be made relatively small. Furthermore, the amount of the movement of the driving gear can be made very small whereby the driving gear can be prevented from being broken.
In accordance with a third invention, an electric power steering apparatus as set forth in the first invention is characterized in that a stopper for limiting the amount of the movement of the bearing in a direction wherein the force for restraining the movement of the bearing is small is provided.
In the third invention, when the driving gear is pressed so as to separate from the driven gear in the first radial direction Y by a meshing reaction force applied to the meshing portion of the driving gear and the driven gear, the elastic members are deflected and the stopper makes contact on the housing side, for example. This can prevent the bearing from excessively moving in the first radial direction Y, whereby the elastic restoring forces of the elastic members in the first radial direction Y can be made further smaller. Hence, the rotation torque applied to the meshing portion of the driving gear and the driven gear can be made further smaller.
In accordance with a fourth invention, an electric power steering apparatus as set forth in the first invention is characterized in that the elastic member is a circular disc having through holes at a plurality of peripheral positions.
In the fourth invention, the elastic member is disposed so that the through hole portions are positioned in the first radial direction Y. Hence, the amount of the backlash can be reduced by the elastic restoring force of the elastic member. In addition, rotation torque applied to the meshing portion of the driving gear and the driven gear can be made relatively small. Furthermore, the amount of the movement of the driving gear in the second radial direction X can be made very small, whereby the driving gear can be prevented from being broken. Moreover, since the elastic member is a disc, the durability of the elastic member can be raised.
In accordance with a fifth invention, an electric power steering apparatus as set forth in any one of the first to fourth inventions is characterized by comprising: an inner side ring fitted onto the bearing; and an outer side ring fitted into the housing; wherein the elastic member is joined to the inner side ring and the outer side ring.
In the fifth invention, the inner side ring, the outer side ring and the elastic member are integrated. Hence, by fitting (pressure-fitting) the outer periphery of the bearing into the inner periphery of the inner side ring and then by fitting the outer side ring into the housing, the bearing and the elastic member can be assembled inside the housing. Assembly workability is thus improved.
An electric power steering apparatus in accordance with a sixth invention comprises a driving gear rotatably supported by bearing inside a housing and rotating interlocked with the rotation of a steering assist motor, and a driven gear meshing with the driving gear and connected to a steering mechanism so that steering is assisted by the rotation of the motor, characterized by comprising: an inner side ring fitted onto the bearing; an outer side ring fitted into the housing; and an elastic member joined to both the inner side ring and the outer side ring to make the driving gear eccentric to the driven gear.
In the sixth invention, since the driven gear can be made eccentric to the driving gear by the elastic member, the amount of the backlash can be reduced. In addition, in the sixth invention, the elastic member itself is not fitted or secured but joined to the inner side ring and the outer side ring, and the inner side ring is fitted onto the bearing and the outer side ring is fitted into the housing. Hence, a preliminary pressure in the eccentric direction by the elastic member can be set in an appropriate range easily. In other words, although rotation torque applied to the meshing portion of the driving gear and the driven gear increases when the preliminary pressure in the eccentric direction is large, and backlash is liable to generate owing to the change of the meshing condition with the passage of time when the preliminary pressure in the eccentric direction is small, these can be prevented by the sixth invention.
Furthermore, in the sixth invention, by fitting (pressure-fitting) the outer periphery of the bearing into the inner periphery of the inner side ring and then by fitting the outer side ring into the housing while the inner side ring, the outer side ring and the elastic member are integrated, the bearing and the elastic member can be assembled inside the housing. Assembly workability of the elastic member is thus improved.
An electric power steering apparatus in accordance with a seventh invention comprises a driving gear rotatably supported by bearing inside a housing and rotating interlocked with the rotation of a steering assist motor, and a driven gear meshing with the driving gear and connected to a steering mechanism so that steering is assisted by the rotation of the motor, characterized by comprising: a ring fitted into the housing; and an elastic member joined to the ring and the bearing to make the driving gear eccentric to the driven gear.
In the seventh invention, since the driving gear can be made eccentric to the driven gear by the elastic member, the amount of the backlash can be reduced. In addition, in the seventh invention, the elastic member itself is not fitted or secured but joined to the ring, and the ring is fitted into the housing. Hence, a preliminary pressure in the eccentric direction by the elastic member can be set in an appropriate range easily.
Furthermore, in the seventh invention, by fitting the ring into the housing while the bearing, the ring and the elastic member are integrated, the bearing and the elastic member can be assembled inside the housing. Assembly workability of the elastic member is thus improved.
In accordance with an eighth invention, an electric power steering apparatus as set forth in the sixth or seventh invention is characterized in that the bearing is rolling bearing, and that the axial position of the driving gear is set at a position wherein a thrust load is applied from the driving gear to the elastic member via the rolling bearings and the elastic member is deflected by the thrust load.
In the eighth invention, since the elastic member for making the driving gear eccentric to the driven gear is deflected in the direction of the thrust load and the elastic member applies the thrust load to the driving gear, it is possible to prevent the driving gear from rattling in the axial direction. Furthermore, in the eighth invention, the thrust load is applied elastically. Hence, the rotation torque of the driving gear can be made small, and the frictional resistance thereof can be reduced.
In accordance with a ninth invention, an electric power steering apparatus as set forth in the eighth invention is characterized in that the elastic member is a circular disc.
In the ninth invention, the radial dimension of the elastic member can be made relatively large. Hence, the elastic member can be deflected easily in the direction of the thrust load, and the thrust load applied from the elastic member to the driving gear can be made small easily.
In accordance with a tenth invention, an electric power steering apparatus as set forth in the ninth invention is characterized by further comprising a positioning member for adjusting the axial position of the driving gear.
In the tenth invention, the elastic member can be deflected by adjusting the axial position of the driving gear by operating the positioning member. Hence, an appropriate thrust load can be applied to the driving gear.
In accordance with an eleventh invention, an electric power steering apparatus as set forth in the tenth invention is characterized in that the positioning member is a screw ring screwed into the housing, that the bearings comprising one pair each fitted onto one of both end portions of the driving gear, respectively, that one of the bearings is held inside the housing via the elastic member and the other bearing is disposed at a position facing the screw ring, and that an elastic ring is provided between the other bearing and the screw ring.
In the eleventh invention, a thrust load is applied from the other bearing to the driving gear via the elastic ring by rotating the screw ring. Furthermore, the elastic member is deflected in the thrust load direction via the one bearing. Hence, the elastic member and the elastic ring apply the thrust loads to the driving gear elastically. As a result, the rotation torque of the driving gear can be made small, and the frictional resistance thereof can be reduced.
An electric power steering apparatus in accordance with a twelfth invention comprises a driving gear rotatably supported by bearing inside a housing and rotating interlocked with the rotation of a steering assist motor, and a driven gear meshing with the driving gear and connected to a steering mechanism so that steering is assisted by the rotation of the motor, characterized by comprising: an inner side ring fitted onto the outer periphery of the bearing; an outer side ring fitted into the housing; and an elastic member joined to both of the inner side ring and the outer side ring to make the driving gear eccentric to the driven gear, wherein a stopper is provided on one of the inner side ring or the outer side ring to limit the amount of the deflection of the elastic member by making contact with another of the inner side ring or the outer side ring, or the housing.
In the twelfth invention, the elastic member is provided on the outer periphery of the bearing for supporting the driving gear inside the housing. Hence, the amount of the backlash at the meshing portion can be reduced by the elastic restoring force of the elastic member. In addition, rotation torque applied to the meshing portion of the driving gear and the driven gear can be made relatively small. Furthermore, in the twelfth invention, when the driving gear is separated from the driven gear in the radial direction by a meshing reaction force applied to the meshing portion of the driving gear and the driven gear, the stopper makes contact with the inner side ring or the outer side ring, or the housing. Hence, it is possible to limit the amount of the deflection of the elastic member. As a result, the driving gear can be prevented from moving beyond its allowable amount of eccentricity, and the meshing condition of the two gears can be made proper.
In accordance with a thirteenth invention, an electric power steering apparatus as set forth in the twelfth invention is characterized in that the stopper has the shape of a disc extended in the radial direction.
In the thirteenth invention, the stopper can be installed inside the housing without considering the position of the stopper with respect to the driven gear. Hence, the elastic member joined to the inner side ring and the outer side ring can be assembled easily. Assembly workability is thus improved.
An electric power steering apparatus in accordance with a fourteenth invention comprises a driving gear rotatably supported by bearing inside a housing and rotating interlocked with the rotation of a steering assist motor, and a driven gear meshing with the driving gear and connected to a steering mechanism so that steering is assisted by the rotation of the motor, characterized by comprising: an inner side ring fitted onto the outer periphery of the bearing; an outer side ring fitted into the housing; and an elastic member joined to both of the inner side ring and the outer side ring to make the driving gear eccentric to the driven gear, wherein the driving gear has an extended shaft portion extended outward from the support portion supported by the bearing inside the housing, and a stopper is provided on at least one of the extended shaft portion or the housing to limit the amount of the deflection of the elastic member by making contact with another of the extended shaft portion or the housing.
In the fourteenth invention, the elastic member is provided on the outer periphery of the bearing for supporting the driving gear inside the housing. Hence, the amount of the backlash at the meshing portion can be reduced by the elastic restoring force of the elastic member. In addition, rotation torque applied to the meshing portion of the driving gear and the driven gear can be made relatively small. Furthermore, in the fourteenth invention, when the driving gear is separated from the driven gear in the radial direction by a meshing reaction force applied to the meshing portion of the driving gear and the driven gear, the stopper makes contact with the extended shaft portion or the housing. Hence, it is possible to limit the amount of the deflection of the elastic member. As a result, the driving gear can be prevented from moving beyond its allowable amount of eccentricity, and the meshing condition of the two gears can be made proper. Furthermore, since the stopper is provided independently of the inner side ring and the outer side ring, when the elastic member is bonded by vulcanization to the inner side ring and the outer side ring, bonding work by vulcanization can be carried out easily on both sides of the inner side ring and the outer side ring without being obstructed by the stopper. Workability can thus be improved.
In accordance with a fifteenth invention, an electric power steering apparatus as set forth in the fourteenth invention is characterized in that the housing has a recess hole into which the extended shaft portion is inserted, and that the stopper is a ring attached to at least one of the extended shaft portion or in the recess hole.
In the fifteenth invention, the ring can be made of a material having small frictional resistance. Hence, the rotation performance of the driving gear can be made proper while the stopper makes contact.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.